1. Field of the Invention
The present invention relates generally to light emitting devices with a light emitting diode (LED) and methods of manufacturing the same and in particular to light emitting devices with a low-cost, high-performance ZnSe homoepitaxial LED and methods of manufacturing the same.
2. Description of the Background Art
Conventionally, AlGaAs and GaAsP for red color, GaP for yellow-green color, AlGaInP for orange and yellow colors, and the like have been used as materials for LEDs of high luminance. Such LEDs are formed on a conductive substrate.
FIG. 4A shows a light emitting device 1a having the LED as described above. As shown in the figure, light emitting device 1a includes resin 2, an LED chip 3a, lead frames 4a and 4b, and a wire 5.
FIG. 4B is an enlarged view of a region 6 shown in FIG. 4A. As shown in the figure, LED chip 3a is connected to lead frame 4a via silver paste 18. LED chip 3a includes a conductive substrate 7a, an epitaxial, light emitting layer 8a causing an emission, a first electrode 10a, and a second electrode 17. The first electrode 10a is connected to lead frame 4b via wire 5.
The LED thus configured is provided by an established, mass-production process at low cost and one such LED is produced at as low a cost as ten yen or therebelow.
However, an LED based on GaInN, a material for blue and green colors and furthermore for white color, is configured as shown in FIGS. 5A and 5B, since it uses as its substrate an insulating sapphire substrate.
More specifically, as shown in FIGS. 5A and 5B, an LED chip 3b has first and second electrodes 10a and 17a on an outermost surface of an epitaxial, light emitting layer 8b. The second electrode 17a is provided in a concave portion formed on a surface of epitaxial light emitting layer 8b. The second electrode 17a is connected to lead frame 4a via a wire 5b and the first electrode 10a is connected to lead frame 4b via a wire 5a. An insulating substrate 7b is fixed to lead frame 4a with silver paste 18 posed therebetween.
Because of the complicated structure as described above, a light emitting device having a GaInN-based LED requires a high manufacturing cost and its unit price is several times greater than the aforementioned low-cost product.
A ZnSe-based material is promising as a material for an LED for blue and green colors and white color. The present inventors have worked on developing an LED having a ZnSe-based homoepitaxial structure employing an n-type ZnSe substrate which is conductive and also transparent in the visible range. Such structure is different from a GaInN-based structure in that it has a conductive substrate. As such, it may have the low-cost LED structure as shown in FIGS. 3A and 3B and theoretically an LED can be manufactured at low cost.
However, it has been found that forming an ohmic electrode on an n-type ZnSe substrate having low carrier concentration requires a special technique. More specifically, for an n-type ZnSe substrate having a carrier concentration of no less than 3xc3x971018 cmxe2x88x923, an ohmic electrode can be readily provided by normally depositing and then thermally annealing with Ti, Al or a similar material. It has been found that for an n-type ZnSe substrate having a carrier concentration less than 3xc3x971018 cmxe2x88x923, however, an ohmic electrode cannot be readily provided by the combination of normal deposition and thermal anneal.
It has also been found that silver (Ag) readily diffuses into a ZnSe crystal and thus readily causes a defect referred to as non-luminescent center. In other words, using silver paste to fix a ZnSe crystal may degrade an LED containing the ZnSe crystal.
The present invention has been made to overcome such disadvantages as above and it contemplates providing an ohmic electrode on an n-type ZnSe substrate having low carrier concentration, and preventing an LED from the degradation as described above.
In accordance with the present invention a light emitting device includes an n-type ZnSe substrate, an electrode base, and a conductive layer. The conductive layer is formed of In or an In alloy and it fixes the n-type ZnSe substrate and the electrode base and also serves as an ohmic electrode for the n-type ZnSe substrate. The electrode base is only required to be formed of conductive material, such as a lead frame or an electrode provided on an insulating substrate.
The present inventors have studied materials for fixing an n-type ZnSe substrate and an electrode base and have found that a conductive layer of In or In alloy may be used as a material for fixing the n-type ZnSe substrate and the electrode base to obtain ohmic contact if the substrate has a carrier concentration of as low as less than 3xc3x971018 cmxe2x88x923. Furthermore, unlike silver, In or the In alloy does not diffuse easily into a ZnSe crystal to cause a defect referred to as non-luminiscent center. As such, an LED can be prevented from degrading due to such diffusion.
In the present invention a light emitting device preferably includes a ZnSe homoepitaxial light emitting diode. Such light emitting diode has on an n-type ZnSe substrate an epitaxial light emitting layer of ZnSe-related compounds.
The present invention is particularly useful for a light emitting device including a ZnSe homoepitaxial light emitting diode capable of emitting blue and green lights and also white light.
The n-type ZnSe substrate has a carrier concentration preferably of at least 3xc3x971017cmxe2x88x923 and less than 3xc3x971018 cmxe2x88x923.
A conductive layer of In or In alloy allows an ohmic electrode to be provided for carrier concentrations as low as above.
In the present invention a light emitting diode preferably operates on a voltage of no more than 3V. Accordingly, it is useful for a backlight of an LCD (Liquid Crystal Display) incorporated for example in a mobile phone.
In the present invention a light emitting device may be manufactured by a method including the following steps: an epitaxial light emitting layer is provided on an n-type ZnSe substrate. In or an In alloy is melted on an electrode base. Directly on the melted In or In alloy the n-type ZnSe substrate is mounted and subjected to at least one of vibration and pressure. Thereafter it is thermally annealed.
In and the In alloy have a melting point as low as 155xc2x0 C. or therebelow and In and the like have a superior wettability with respect to metal and also solidify at room temperature. As such, In and the like can be used as a conductive adhesive or so-called solder. The present inventors have studied whether In and the like can be used for adhering the n-type ZnSe substrate and the electrode base together and found that using the technique characteristic to the invention as described above allows In and the like to be used as a conductive adhesive in accordance with the present invention. More specifically, by mounting the n-type ZnSe substrate directly on melted In or the like and subjecting it to at least one of vibration and pressure the In or the like can diffuse into the ZnSe crystal substrate for a temperature as low as approximately 200xc2x0 C., and thereafter by thermally annealing the same an eutectic alloy can be produced and ohmic contact can be obtained for the ZnSe substrate of low carrier concentration.
The vibrant applied is preferably ultrasonic vibration. The pressure applied is preferably at least 0.544 MPa (5.56xc3x9710xe2x88x922 kg/mm2) and less than 109 MPa (11.1 kg/mm2).
The vibration applied may be a scrubbing operation caused by a mechanical vibration having a frequency of at least 1 Hz and at most 1000 Hz. In this instance, a pressure of at least 0.217 MPa (2.22xc3x9710xe2x88x922 kg/mm2) and less than 109 MPa (11.1 kg/mm2) is applied simultaneously with the vibration.
If vibration and pressure are applied simultaneously, preferably the frequency is at least 10 Hz and at most 300Hz and the pressure is at least 0.217 MPa (2.22xc3x9710xe2x88x922 kg/mm2) and at most 10.9 MPa (1.11 kg/mm2). More preferably, the frequency is at least 10 Hz and at most 60 Hz and the pressure is at least 0.217 MPa (2.22xc3x9710xe2x88x922 kg/mm2) and at most 5.45 MPa (0.555 kg/mm2).
By applying such vibration and/or pressure to the n-type ZnSe substrate, ohmic contact can be obtained, as described above.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.